New entertainment and communication services are today available to commercial or business airline passengers, these services being routinely combined under the designation “In-Flight Entertainment” (hereinafter called IFE). This can be, for example, high speed Internet access or even a video on demand service for each passenger. Thus, each passenger can, for example, choose to individually view any video content, such as a film. By an appropriate interface, each passenger can select what he wants to view. The film starts almost instantaneously, and it is not even necessary to wait for the film to download, which would represent a fairly long time. Subsequently, he can temporarily interrupt the viewing then resume it, or rewind to review a sequence, or even fast forward to skip sequences, once again with the same instantaneity as the start of the display. For the passenger, everything happens as if he were at home in front of a television connected to a video disc player.
This type of service is now available on board many airplanes. The current systems use in particular display modules at seatback level, input modules at armrest level, player and transmission modules at cabin head level, all these modules being interconnected by wired links. The IFE systems have consequently exploded the quantity of wiring for each seat, to the detriment of the configurability of the cabin.
Now, the configurability of the cabin is an important marketing factor for aircraft manufacturers, in particular the possibility for the customers to choose the number and the layout of the seats. In practice, given a few safety constraints, the airlines are relatively free as to the interior layout of the cabin when they order an airplane. This is so that they can adapt the cabin to the type of clientele on the line that they are expecting to operate. The IFE wiring of the seats has therefore become an increasingly weighty brake on the multipurpose nature of the aircraft. However, in the same way, a complete and efficient IFE system has also become a major marketing trump card for the aircraft manufacturers. The increased competition in the field of air transport and the democratization of airplane travel that has resulted therefrom, have made the IFE services, which could have seemed accessories in their beginnings, indispensable in winning market shares. It has therefore become appropriate to reconcile the technical constraints of the IFE systems with an optimum configurability of the cabin.
Thus, being aware of the growth of the wireless information transfer technologies, certain aircraft manufacturers have, in agreement with the airline operators and IFE system providers, quite simply eliminated certain IFE cables going to the seats, that are among the biggest hindrances to the configurability of the cabin. Only cables linking seats in small groups of two, three or four seats have been retained. It has become the responsibility of the IFE system providers to adapt, particularly by exploiting to the maximum the remaining wiring and making the best use of wireless technologies.
Developing a wireless communication protocol dedicated to video on demand in an IFE system would no doubt have led to a very efficient solution, but this would no doubt also have been very expensive. This is why the use of a communication standard has been preferred. Unfortunately, none of the current standards satisfies all the issues of video on demand in an IFE system. However, given in particular the maturity of the different technologies, the IEEE 802.11a, IEEE 802.11b, IEEE 802.11g or IEEE 802.11n standards, better known under the commercial designation of “WiFi links”, appear the best suited. They have consequently been retained. In practice, the WiFi links give results that are quite satisfactory in consumer video applications, but applications without multiple broadcast constraint or display instantaneity constraint. Hereinafter, they will also be combined under the generic designation of 802.11.
In practice, on board a long haul airplane, several dozen or even several hundreds of passengers can simultaneously view different films, each passenger being able to interrupt, fast forward or rewind at will in the film that he is viewing. Consequently, each film viewed by a passenger constitutes a particular session, the corresponding video data stream having to be sent exclusively to the display screen of a single passenger. For example, an IFE system can broadcast 300 sessions simultaneously. Such a multiple broadcast constraint does not exist in any current consumer WiFi link video application. Similarly, the current video applications of the WiFi link do not provide almost instantaneous display, particularly on starting up the film. Waiting times of several seconds, sometimes masked by advertising or warning messages, are necessary before a play command can actually be executed. Thus, the applications that use a WiFi link in the home normally exploit the IEEE 802.11b and IEEE 802.11g standards, which provide three channels in the 2.4 gigahertz region for a total bit rate that can range up to 20 megabits per second (Mbps). A small number (a few units) of video streams can be sent simultaneously by each channel. This is not sufficient in the context of video on demand in an IFE system, for which the IEEE 802.11a standard seems better suited. The latter provides 23 channels between 5 and 6 gigahertz for a total bit rate that can range up to 600 Mbps. However, the IEEE 802.11a standard is even so not perfectly suited to the needs of video on demand in an IFE system.
In practice, the IEEE 802.11a standard does not guarantee that a packet will be received, or even guarantee that a packet will be received at a reasonable cost. Because the 802.11 standards are based on an acknowledgement mechanism: a message called ACK, an abbreviation of “ACKnowledgement”, is sent to a sending WiFi device for each data packet received by a receiving WiFi device. The sending WiFi device sends the packet again to the receiving WiFi device until it receives the corresponding ACK message. After a certain number of re-sendings, the packet is no longer sent and it is definitively lost for the receiving WiFi device. By this mechanism, besides the fact that the bandwidth is used up in sending the ACK messages, it should be noted that, to recover a first lost packet, possibly many other packets can be lost subsequently! Thus, if the error rate is high, that is, if the number of packets lost compared to the number of packets sent is high, a delay can easily build up and errors can appear on the screen. The 802.11 standards respond to this by reducing the bit rate, which is not acceptable in the multiple video on demand context. An essential characteristic of the standard WiFi links is to provide an ongoing trade-off between the error rate and the bit rate. If the error rate increases, the bit rate reduces and vice-versa. It should, moreover, be noted that the data packets can be definitively lost and the data ultimately displayed can contain errors, which are manifested in most cases in black pixels or refresh failures. Some display terminals attempt to attenuate the visual discomfort that these residual errors can provoke, by image smoothing methods for example. Ensuring a constant rate for a WiFi link is one of the technical problems to which the invention proposes to provide a solution.
In the relatively cramped cabin of an airplane, even one the size of a long haul airplane, the errors are not due to the distance between the sending WiFi device and the receiving WiFi device. The problem comes from a conventional phenomenon known as “microfading”: at very high frequency, the reception level can vary in time and in space. In the present case, the “microfading” is explained on the one hand by a phenomenon of resonance of the water molecules at the frequency used, the water being very present in the body of each of the many passengers in constant motion, and on the other hand by a phenomenon of wave reflection on metallic objects, metal being omnipresent in the cabin of an airplane.
The WiFi technology, initially designed to bring the Internet into the home, exchange emails and download files with no real time constraint, was not designed to be robust faced with the “microfading” phenomenon and provide a constant bit rate. Some more recent 802.11 standards provide for a quality of service in terms of latency, error rate and bit rate. However, they rely on a buffering mechanism of at least ten or so data seconds, which is incompatible with the almost instantaneous display demanded by video on demand in an IFE system, particularly on starting. The needs of video on demand in an IFE system do not therefore definitively tally with the development constraints of the WiFi technology. This is one of the reasons why multiple video on demand does not currently exist in a wireless IFE system. Today, a cabin head player module is still linked to the screens by an end-to-end wired link passing through the seats. The present invention proposes to resolve the problem of non-constant bit rate on a WiFi link in order to enable video on demand to be implemented in a wireless IFE system.
In closely related fields such as video on demand via an ADSL (Asymmetric Digital Subscriber Line) link, the problem of non-constant bit rate has been resolved by putting in place, at the network layer TCP/IP level, a buffering mechanism of a sufficiently long video stream duration. The volume of data contained in the buffer memory varies in proportion to the download rate. If the download rate is kept at a sufficiently high level for the buffer memory never to be emptied, then the continuity of the displayed video stream is assured. Otherwise, the image is frozen when the buffer memory is empty, it moves again only when the bit rate returns to a sufficient level for the buffer memory to be filled once again. One advantage of this solution is that the content of the buffer memory contains no error and the display is perfect with no correction of the display terminal. However, a major drawback of this solution is that the bit rate can vary and that the image can be frozen when the link is particularly bad.